Also see Growth Promotants and Production Enhancers.
There are 2 groups of performance modifiers used in beef cattle production—hormone-like growth enhancers (mostly subcutaneous implants) and ruminal chemistry modifiers, which alter volatile fatty acid production in the rumen.
Hormone-like growth enhancers for finishing beef cattle were discovered in 1948, when it was shown that subcutaneous implantation of diethylstilbestrol (DES) caused an increase in growth rate of ~10%. The use of DES in finishing beef cattle was practiced for ~25 yr, until the FDA declared it illegal. However, other similar compounds were approved by the FDA by that time. The average increase in daily gain due to the use of effective implants is ~0.23 kg (0.5 lb), and the improvement in the feed to gain ratio is 0.56 kg feed/kg gain.
Ionophores are antibiotics that alter the chemistry of the rumen by altering the rumen microflora to produce increased proportions of propionic acid and decreased proportions of acetic and butyric acids. These 3 acids, called volatile fatty acids (VFA), are the products of ruminal fermentation and can be absorbed from the digestive tract of the cow and used as energy sources. Because propionic acid releases more energy per unit weight to the host cow upon oxidation than do the other 2 VFA, it is important in beef cattle production. However, because butyric acid, especially, is involved in butterfat production, anything that decreases its proportions in ruminal fermentation is undesirable. Therefore, ionophores are not used in lactating dairy cattle management.
In a recent summary of 67 finishing trials and 55 pasturing beef cattle trials, ionophores improved feed:gain ratio and mean daily weight gain (see Nutrition: Cattle: Effects of Ionophores on the Performance of Beef Cattle).
The consistent improvement in gain of cattle fed ionophores on high-roughage diets as compared with less consistent improvement for cattle fed high-energy diets is explained by energy availability between the 2 systems. On high-energy diets, cattle eat until they meet energy requirements; thus, ionophores help them derive more energy per unit of ingested feed, and they eat less. On high-roughage diets (less energy per unit weight), cattle consume feed until the rumen will hold no more. However, on the latter program, if ionophores are fed, such cattle derive more energy per unit of feed consumed, and thus gain more.
Last full review/revision July 2011 by Thomas H. Herdt, DVM, MS, DACVN, DACVIM; Tilden Wayne Perry, BEd, BS, MS, PhD